1. Field of the Invention
This invention relates to the manufacture of integrated circuits and more particularly to the characterization and adjustment of steppers used for patterning integrated circuits on wafers.
2. Background of Related Art
The manufacture of integrated circuits involves multiple patterning steps that are principally carried out by devices known as steppers. These steppers typically employ step and repeat systems, whereby a mechanical stage is utilized to move a projection field relative to an exposure target. This motion is carried out in fixed increments across a two dimensional wafer surface with an exposure being made at each increment. Steppers using step and repeat systems are exemplified by products like the Nikon NSR-2005i9C, the Canon FPA2000il, the ASM PAS 2500/40, and the Ultratech 2244i.
An alternative to steppers using step and repeat systems are steppers utilizing step and scan systems, exemplified by the SVGL Microscan II. The step and scan methodology specifies a stepper that moves the projection equipment in fixed increments, like a step and repeat system, but rather than exposing an entire field at once, a step and scan system scans across the exposure field with a narrow exposure window.
A stepper is composed of the following: 1) an illumination source of any type, including but not limited to ultraviolet light, x-rays, and electron beams; 2) a mask or reticle, upon which is a circuit pattern for the semiconductor device to be manufactured; 3) a lens assembly, composed of any number of elements, whose purpose is to project a real or latent image upon the wafer; 4) a stage, upon which is carried a target device plate or wafer; and 5) a controller for controlling the operation of the stepper.
In a typical integrated circuit (IC) fabrication process, different layers are patterned using multiple steppers--each stepper responsible for patterning a single layer on each product die. In a homogeneous manufacturing environment, all the steppers used to create a single product are of similar design. This generally results in each stepper producing a field image whose size is equal to or approximately equal to the field image size of the other steppers used in a process.
In a heterogeneous manufacturing environment (also known as mix and match), steppers of differing design are combined to create a single product. The field image sizes of these steppers might differ significantly. In some cases, a stepper design might be capable of imaging more than double the field area of another stepper design. This can lead to the possibility of using 2:1 field matching to increase product throughput. Referring to FIG. 1, in 2:1 field matching the wide field stepper will overlay a single wide field 10 such that it encompasses two narrow fields 12,14.
One task that is required to produce increased yields of working ICs is the calibration or matching of steppers in a production line. This matching typically involves a combination of hardware and software adjustments to the steppers. A representative list of possible adjustments include:
--Wafer rotation PA1 --Stage motion scaling PA1 --Stage motion orthogonality PA1 --Mirror mapping PA1 --Translation offset PA1 --Magnification PA1 --Reticle rotation (intrafield rotation) PA1 --Field tilt.
The above adjustments are routinely carried out using embedded alignment and metrology systems which are part of the individual stepper systems. These embedded systems have limited ability to compensate for pattern misregistration due to the following reasons: 1) the number of intrafield and wafer sites observed by the alignment system is small such that there could exist significant difference between the observed or estimated misregistration and the true misregistration; and 2) each system is entirely embedded within a single stepper and is incapable of making or reporting adjustments to other steppers.
The above limitations are removed through the use of an independent overlay metrology system, such as the Prometrix EM-1, the IVS Accuvision, the KLA 5011, the OSI Metra, and the BioRad Quaestor.
An overlay metrology tool is composed of the following: 1) a system for loading one or more previously patterned wafers into a handling system; 2) a handling system that positions individual wafers relative to a measurement system; 3) a measurement system that measures the overlay misregistration at a number of locations on the wafer; and 4) a system for recording, storing, and/or transmitting the measurement information. Mechanisms for performing the overlay measurements include, but are not limited to, 1) analysis of optical images of the patterned wafer surface observed through a microscope and recorded using a digital imaging system; and 2) differential electrical resistance measurements obtained via physical probing of structures on the wafer surface.
Patterning can be accomplished using a blind-step technique, whereby the stepper places a pattern on a blank wafer absent of alignment marks. Alternatively, if alignment marks are present on a previously patterned layer, a stepper can utilize an alignment system to pattern a new layer which is aligned to the layer upon which the marks reside.
In step and repeat systems and in step and scan systems the stage is responsible for moving the wafer in a precisely stepped pattern such that individual dies can be illuminated by the patterning field. The number of dies on a wafer is a function of the die size, the size of the wafer, and the organization of dies. The size of the patterning field on a stepper is typically equal to or greater than the size of the dies upon which the image is being patterned.
In a homogeneous fabrication environment (same steppers design), the field sizes of the steppers are all approximately the same. The result is that each stepper is only able to pattern a single die at each step position.
2:1 field matching is possible when the pattern field areas on one or more steppers in a production line are at least twice as large as the dies that are being imaged. Typically, this requires moving to a heterogeneous stepper line by mixing different design steppers that have different maximum field sizes. An example of such mixing is an Ultratech 2244i 1X stepper mixed with current reduction steppers from Nikon, Canon, and ASM. The Ultratech 2244i has a maximum field size of 22 mm.times.44 mm on the wafer. Current reduction steppers have maximum field sizes on the order of 22 mm.times.22 mm. This leads to the possibility of using the Ultratech 2244i to simultaneously pattern two reduction fields.
The characterization of assignable overlay sources for homogeneous 1:1 stepper matching has been analyzed and applied in a number of forums. Utilization of these 1:1 models in a 2:1 scenario has yielded sub-optimal results that are extremely difficult to relate to the true assignable error sources. The system according to the present invention characterizes 2:1 stepper matching such that the end result of an analysis can be directly related to actual assignable sources.